Flash distillation with condensed refrigerant as heat exchanger



United States Patent 3,461,460 FLASH DISTILLATION WITH CONDENSEDREFRIGERANT AS HEAT EXCHANGER William L. McGrath, Syracuse, N.Y.,assignor to Carrier Corporation, Syracuse, N .Y., a corporation ofDelaware Filed Oct. 21, 1965, Ser. No. 499,845 Int. Cl. C02b 1/06; 301d3/06 US. Cl. 203-11 13 Claims ABSTRACT OF THE DISCLOSURE A system forflash distillation of sea water to form potable water having a flashevaporator, a refrigerated flash evaporator, and an additional flashevaporator, a compressor, condenser and refrigerant evaporator disposedin the refrigerated flash evaporator, and a boiler, turbine and turbinesteam condenser. Sea water is passed through condensing sections in theflash evaporator thence through the refrigerant condenser and theturbine steam condenser to heat the incoming sea water, which is thenpassed backwardly through the evaporator sections of the flashevaporator, the refrigerated flash evaporator, and the additional flashevaporator, where the heated sea water is successively flashed to formwater vapor. The water vapor is condensed in the condensing sections ofeach of the flash evaporators to form product water. Heat is removedfrom the condensing section of the refrigerated flash evaporator andpumped to the incoming sea water through the refrigerant condenser viathe refrigerant compressor. The condensed refrigerant is then passedthrough heat exchanger in the evaporating sections of the flashevaporator to subcool the refrigerant and assist in flashing water fromthe solution in the evaporating sections.

This invention relates to separation systems and more particularly toseparation systems which employ a refrigeration cycle and are adaptedfor separating a solvent from a solution. Systems of the type hereindescribed are particularly useful for converting sea water to potablewater and the invention will be described in that context.

Among the various types of saline water conversion systems which havebeen proposed, the so-called flash distillation process, has foundconsiderable favor due to its compactness and its theoretically,potentially low operating costs.

However, attempts to provide large scale potable water production by theflash distillation process have generally resulted in proposals havingan excessive initial cost due to the enormous heat transfer surfacerequired to make the plants operating cost favorable.

In spite of this disadvantage, the availability of large quantities oflow cost steam from atomic power generation plants made the flashdistillation process especially attractive for large size saline waterconversion plants. Any increase in efliciency of the flash distillationcycle which results in a decrease in the operating costs of the planthas a significant effect on the economic attractiveness of the overallsystem.

One way in which the cost of obtaining potable water can be materiallyreduced is by providing a refrigeration cycle to absorb low-grade heatfrom eflluent of the still or from any other convenient heat source at afavorable temperature level and convert it into high temperature heatwhich is then added to the sea water input to the still. By this means,the steam requirement and therefore the energy cost for producing freshwater can be cut approximately in half and the cost of the' overallplant can also be reduced since the size of the steam generationequipment is thereby reduced.

Various additional improvements in the saline water conversion apparatuscan be employed to reduce the cost of operation of the still. However,such improvements normally result in adding a number of heat exchangers1n the system or otherwise increasing the heat transfer surfaceelsewhere in the system, thereby eliminating the advantage gained inreducing the operating costs of the system.

For example, it has been previously proposed to utilize a steam driventurbine to operate the compressor of a heat pump, as previouslydescribed, and to condense the exhaust steam from the turbine in a heatexchanger which supplied heat to operate a second flash distillationstill. While such a system is moderately eflicient from a thermodynamicpoint of view, the system is complex and requires two major flashdistillation stills, yet does not provide optimum efliciency.

Accordingly, it is a principal object of this invention to provide animproved apparatus and method for separating solvent from a solution bymeans of a refrigeration cycle.

It is a further object of this invention to provide a relatively simple,inexpensive, and thermodynamically highly efficient separation systemand method of separation which employs a refrigeration cycle and isespecially suited for rendering saline water potable.

These objects are achieved in a preferred embodiment of this inventionby providing a saline water conversion plant having a main flashevaporator, a refrigerant evaporator, a refrigerant condenser, and apower fluid condenser connected to distill fresh water from the salinewater fed to the system. A heat pump is employed to absorb heat from asuitable portion of the system such as by condensing fresh water and toreject that heat at a higher temperature to the sea water introducedinto the evaporating sections of the flash evaporator. Sea water isintroduced into the system through a heat exchanger in the condensingsections of the flash evaporator where it absorbs heat by cooling andcondensing water vapor to provide fresh water product from the system.The heated sea water passes from the condensing sections through therefrigerant condenser and power fluid condenser where it absorbs heat bycooling and condensing refrigerant and power fluid respectively. Theheated sea water is then introduced into the evaporating sections of theflash evaporator where the sea water is flashed to provide water vaporto the respective condensing sections. In this embodiment of theinvention refrigerant is evaporated in the condensing section of arefrigerated flash evaporation stage where the refrigerant absorbs heatby cooling and condensing water vapor to provide additional fresh water.The refrigerant is then compressed and condensed in the refrigerantcondenser.

In accordance with this invention, the relatively hot condensedrefrigerant is passed from the refrigerant condenser through a heatexchanger in several of the evaporating sections of the main flashevaporator to subcool the refrigerant while at the same time providingadditional heat to increase the quantity of water vapor formed in theflash evaporator, thereby materially increasing the efficiency of thesystem and reducing the cost of obtaining fresh water from it. At thesame time, the use of subcooled refrigerant in the refrigerantevaporator also increases the amount of water vapor condensed for eachpound of refrigerant, thus reducing the power expended in therefrigeration system or increasing its capacity at no increase in power.

The apparatus and method of separating solvent from a solution inaccordance with this invention provides a material decrease in theoperating costs of the system. At

the same time, the system in accordance with this invention does notrequire larger heat transfer surface in the system but actually permitsa reduction in heat transfer surface per unit of output if desired.

The above and other objects of this invention will be more readilyunderstood from the following detailed description and with reference tothe attached drawing wherein the figure is a cross-sectional schematicflow diagram of a saline water conversion plant in accordance with thisinvention.

Referring particularly to the drawing, there is shown a separationsystem for separating a solvent component from a solution. While theapparatus and method to be described has general application, it will beassumed, for purpose of illustrating the best known embodiment thereof,that it is desired to separate fresh or potable water from sea water. Itwill be appreciated, however, that other solvents and other solutes canbe separated by the method and apparatus described, and that the endproduct of the system may comprise either concentrated solution, as in afruit juice concentration process, or may comprise the solvent, as in asaline water conversion process.

The major component of the apparatus illustrated comprises a flashevaporator or still which is divided into a plurality of stages 11, 12,13, and 14 respectively. Each stage of flash evaporator 10 has acondensing section 15, preferably located adjacent the upper portionthereof, and solution evaporating section 16 preferably located adjacentthe lower portion thereof. The condensing section of each stage includesportions of a heat exchanger 17 and the portions of the heat exchangerof each stage are preferably connected in series with the heatexchangers of adjacent stages to provide a continuous path for flow of acooling medium through the condensing sections. Each of the stages offlash evaporator 10 are also provided with condensate pan 18 disposedbelow the heat exchanger for collecting condensate formed thereon. Thecondensate pans of each stage are connected by a condensate line 19 andrestrictor or flow control means 22 to provide a continous path forcondensate to flow through flash evaporator 10. The solution evaporatingsections of all or some of the stages of flash evaporator it} are alsoprovided with portions of a heat exchanger 20, each of which ispreferably connected in series with the portions of the heat exchangerdisposed in adjacent stages. A restricted solution passage 21 isprovided between each of the evaporating sections of flash evaporator 10to provide a continuous path for the proper rate of flow of solutionthrough the flash evaporator. The condensing and evaporating sections ofeach stage are in communication with each other to allow vapor to passfrom the evaporating sections of the condensing sections.

A refrigerated flash evaporator stage 25 has a condensing section 26adjacent the upper portion thereof which is provided with a heatexchanger 27 and a condensate pan 28. Refrigerated flash evaporator 25also includes a solution evaporating section 29 preferably adjacent thelower portion thereof which is in communication with condensing section26.

An additional flash evaporator stage 35 may be employed as shown and hasa condensing section 36 preferably adjacent the upper portion thereof inwhich is disposed a condensate pan 37 and a heat exchanger 38.Additional flash evaporator stage 35 also includes an evaporatingsection 39 adjacent the lower portion thereof in communication withcondensing section 36. It will be understood that additional flashevaporator stage 35 may actually comprise several stages and can becombined, along with refrigerated flash evaporator 25 as added stages offlash evaporator 10. A suitable purge unit of known construction (notshown) is employed to purge the stages, through lines 34, ofnoncondensible gases.

The system also includes a refrigerant condenser 40 having a heatexchanger 1 therein and a power fluid condenser 45 having a heatexchanger 46 therein. A boiler or other source of power fluid vapor 49is provided to supply power fluid vapor to a turbine 50 which isconnected to directly or indirectly operate a refrigerant compressor 51.For example, turbine 50 could be connected to operate an electricalgenerator which in turn could operate an electric motor to drivecompressor 51.

In operation, sea water or other solution from which it is desired toseparate solvent is introduced to the apparatus through inlet line 55.The sea water passes into line 59 and a portion of the sea water passesthrough line 57 where it passes through heat exchanger 38 in additionalflash evaporator stage 35 and is discharged from the system, throughpump 54 and line 58. Sea water also passes from line 59 through pump 56and through the series of heat exchangers 17 in the condensing sectionof the stages of flash evaporator 10.

The relatively cool sea water passing through the condensing sections offlash evaporators 35 and 10 cools and condenses water vapor present inthe condensing section by absorbing heat from the water vapor, therebyheating the sea water during its passage through the heat exchangers.The heated sea water then passes from the condensing sections of themain flash evaporator through the heat exchangers 41 and 46 respectivelyof refrigerant condenser 44 and power fluid condenser 45. In each of thecondensers, the heated sea water, which is relatively cool with respectto the vapor in the condensers, cools and condenses the vapor thereinwhile picking up the heat of condensation to further heat the sea water.

The relatively hot sea water then passes from the refrigerant and powerfluid condensers through restricted line 62 into the evaporatingsections of flash evaporator 10. The hot sea water flashes down to thepressure established by the temperature of heat exchanger 17 in thecondensing section of stage 11, thereby providing water vapor to thecondensing section and cooling the sea water slightly. The slightlycooled sea water then passes from stage 11 of flash evaporator 10through passage 21 into stage 12. The sea water passing through heatexchanger 17 in stage 12 is at a lower temperature than that passingthrough heat exchanger 17 in stage 11. Consequently, the pressure instage 12 of flash evaporator 10 is slightly lower than the pressure instage 11 thereof. For this reason, additional water vapor will flashfrom the solution which has passed from the evaporating section of stage11 to the evaporating section of stage 12. This process of flashingadditional water vapor is repeated in each of the stages of flashevaporator 10. While, for convenience of illustration, flash evaporator10 has been shown as having four stages, in actual practice flashevaporator 10 may have 40 or more stages to optimize the thermodynamicefliciency.

The flash cooled, concentrated, brine solution emerges from the lowestpressure stage 14 of flash evaporator 10 through line 63 from which itis passed into the solution evaporating section 29 of refrigerated flashevaporator stage 25. The pressure in evaporator stage 25 is lower thanthe pressure in the last stage 14 of flash evaporator 10 and,consequently, additional Water vapor is flashed in solution evaporatingsection 29 from which the water vapor passes into condensing section 26of refrigerated flash evaporator 25.

The more highly concentrated brine is then passed from solutionevaporating section 29 through line 64 into evaporating section 39 ofadditional flash evaporator stage 35. Flash evaporator stage 35 is at astill lower pressure than the pressure in refrigerated evaporator 25since the temperature of the incoming solution is lower than thetemperature at which the refrigerant evaporates and additional Watervapor is flashed from the solution in second flash evaporator 35. Theflashed water vapor passes from evaporating section 39 into condensingsection 36.

The relatively highly concentrated brine is discharged from evaporatingsection 39 through line 65. The concentrated brine is split into twoportions by diverting valve 68. One portion of the brine is recirculatedthrough the system by passing through line 67 to inlet line 55. Theother portion of the brine, is discharged from the system. The ratio ofrecirculated brine to discharged brine is adjusted in a manner toprevent excessive concentration of solute in the apparatus. It will beunderstood that a suitable pump or plurality of pumps may be employed atdesired locations to forward solution through the lines connecting thecomponents of the apparatus.

While cool sea water is employed to condense water vapor in thecondensing sections of flash evaporator and 35, refrigerant is employedto condense water vapor in the condensing section of refrigerated flashevaporator 25. Relatively cool liquid refrigerant is supplied fromexpansion valve 74 into heat exchanger or evaporator 27 in condensingsection 26 of refrigerated evaporator 25. The refrigerant passes in heatexchange relation with water vapor in condensing section 26 thuscondensing fresh water at the same time causing the refrigerant toevaporate. While any volatile fluid may be used as a refrigerant,favorable thermodynamic characteristics are found with either methylchloride having the formula CH CI, or dichloromonofluoromethane havingthe chemical formula CHCl F.

The evaporated refrigerant is withdrawn from evaporator 27 through vaporline 70 and is compressed by refrigerant compressor 51. The compressedrefrigerant passes through line 71 to refrigerant condenser 40' Where itis condensed by heat exchange with partially heated sea water throughheat exchanger 41. Thus, heat is pumped from the condensing fresh watereflluent to the sea water input to the evaporating sections of flashevaporator 10.

In accordance with this invention, the relatively warm condensed liquidrefrigerant passes through refrigerant liquid line 72 into the heatexchanger 20 in the evaporator section of one of the stages of flashevaporator 10. The refrigerant liquid passes through successive stagesof the flash evaporator in heat exchange relation with sea water in theevaporating sections thereof. The hot liquid refrigerant is cooled bygiving up heat to the sea water in the evaporating sections of the flashevaporator stages and therefore causes additional generation of watervapor.

The subcooled refrigerant liquid emerges from the last section of heatexchanger 20 in flash evaporator 10 and is passed through line 73 andexpansion valve 74 to heat exchanger 27 in refrigerated flash evaporator25 as previously described. In practice, sections of heat exchanger 20may be present in only some of the stages of flash evaporator 10, ifdesired.

The subcooling of refrigerant liquid has an especially advantageouseffect on cycle etficiency which results in a reduction in the waterproduction costs of the system. This effect is achieved because heat isgiven up from the condensed refrigerant to assist in the generation ofWater vapor in the flash evaporator stages. At the same time, it isthermodynamically desirable to supply subcooled refrigerant to heatexchanger 27, which comprises the refrigerant evaporator coil ofrefrigerated flash evaporator 25 because the amount of water vaporcondensed is thereby increased and the power demand on compressor 51 ismaterially reduced.

A system in accordance with this invention results in a materiallysmaller and less costly saline water conversion or separation apparatusor one which will produce a desired quantity of product at a lesserexpenditure of energy.

The advantages of this invention can be achieved in various physicalarrangements thereof and with other types of distillation apparatus suchas a submerged coil evaporator still. Consequently, while there has beendescribed, for purposes of illustration, a preferred embodiment of thisinvention, it will be appreciated that the invention may be otherwiseembodied within the scope of the following claims.

6 I claim: 1. An apparatus for separating solvent from a solutioncomprising: 3

(A) a flash evaporator having a condensing section and an evaporatingsection, said condensing section having a heat exchanger therein, andsaid evaporating section having a heat exchanger therein;

(B) a refrigerant evaporator comprising a heat exchanger for evaporatingrefrigerant;

(C) a refrigerant condenser comprising a heat exchanger for condensingrefrigerant;

(D) means to pass said solution through the heat exchanger in saidcondensing section of said flash evaporator in heat exchange relationwith solvent vapor therein to heat said solution and to simultaneouslycondense solvent in said condensing section;

(E) means to pass said solution from said condensing section of saidflash evaporator into heat exchange relation with refrigerant vapor insaid refrigerant condenser to further heat said solution and tosimultaneously condense refrigerant vapor in said refrigerant condenser;

(F) means to pass solution heated by heat exchange with condensingrefrigerant into the evaporating section of said flash evaporator toflash solvent vapor therefrom, thereby providing solvent vapor forcondensation in said condensing section of said flash evaporator;

(G) means to pass liquid refrigerant through the heat exchanger in saidrefrigerant evaporator to absorb heat from a suitable source thereof;

(H) means to withdraw refrigerant vapor evaporated in said refrigerantevaporator and to pass the refrigerant vapor to said refrigerantcondenser to condense said refrigerant vapor therein;

(I) means to pass liquid refrigerant from said refrigerant condenserthrough the heat exchanger in the evaporating section of said flashevaporator to simultaneously heat the solution in said evaporatingsection and to subcool said liquid refrigerant; and

(I means to pass said subcooled refrigerant from said flash evaporatorto said refrigerant evaporator for evaporation therein.

2. An apparatus as defined in claim 1 including:

(A) a refrigerated flash evaporator comprising a solution evaporatingsection, and a condensing section having said refrigerant evaporatordisposed therein;

(B) means to pass solution from the evaporating section of said flashevaporator into the solution evaporating section of said refrigeratedevaporator to flash additional solvent vapor from said solution, therebyproviding solvent vapor in the condensing section of said refrigeratedevaporator; and

(C) means to pass subcooled refrigerant to the condensing section ofsaid refrigerated evaporator to condense additional solvent vaportherein by evaporation of said refrigerant.

3. An apparatus for separating a solvent from a solution as defined inclaim 2 including:

(A) an additional flash evaporator stage having a condensing section andan evaporating section;

(B) means to pass solution from the solution evaporating section of saidrefrigerated flash evaporator to the evaporating section of saidadditional flash evaporator stage to flash additional solvent vapor fromsaid solution and to provide solvent vapor in the condensing sectionthereof; and

(C) means to pass a cooling medium through said condensing section ofsaid additional flash evaporator stage to condense additional solventvapor in said condensing section.

4. An apparatus as defined in claim 1 wherein said solution comprisessaline water, said refrigerant comprises methyl chloride, and saidcondensed solvent comprises potable water.

5. An apparatus as defined in claim 1 wherein said solution comprisessaline water, said refrigerant comprises dichloromonofluoromethane, andsaid condensed solvent comprises potable water.

6. An apparatus for separating solvent from a solution thereof asdefined in claim 1 wherein said flash evaporator comprises a pluralityof stages each having a condensing section and an evaporating section.

7. An apparatus for separating solvent from a solution as defined inclaim 1 further comprising:

(A) a refrigerant compressor;

(B) a heat operated prime mover connected to drive said compressor;

(C) said compressor being connected to Withdraw refrigerant vapor fromsaid refrigerant evaporator and to forward compressed refrigerant tosaid refrigerant condenser; and

(D) means to reject heat from said heat operated prime mover to saidsolution to assist in vaporizing solvent therefrom.

8. An apparatus for separating solvent from a solution as defined inclaim 1 further comprising:

(A) a turbine connected to drive a compressor;

(B) said compressor being connected to withdraw refrigerant vapor fromsaid refrigerant evaporator and to forward compressed refrigerant tosaid refrigerant condenser;

(C) a boiler connected to supply power fluid vapor to operate saidturbine;

(D) a power fluid condsnser connected to receive power fluid dischargedfrom said turbine; and

(E) means to pass solution passing from the condensing section of saidflash evaporator to the evaporating section of said flash evaporatorthrough said power fluid condenser in heat exchange relation with powerfluid vapor therein to condense said power fluid vapor and tosimultaneously heat said solution.

9. A method of separating solvent from a solution in an apparatusincluding a solution flash evaporator having a condensing section and anevaporating section, a refrig erant evaporator, a refrigerant condenser,and a refrigerant subcooler which comprises the steps of:

(A) adding heat to said solution by passing it in heat exchange relationwith refrigerant in said refrigerant condenser, thereby simultaneouslycondensing refrigerant in said refrigerant condenser;

(B) vaporing solvent from the solution heated by heat exchange withcondensed refrigerant, by passing said heated solution from therefrigerant condenser through the flash evaporating section of saidsolution evaporator, thereby forming vapor for condensation in saidcondensing section thereof and subcooling the condensed refrigerant bypassing the condensed refrigerant from the refrigerant condenser throughthe refrigerant subcooler in heat exchange relation with the solution inthe flash evaporator;

(C) evaporating refrigerant in said refrigerant evaporator and therebyabsorbing heat from a suitable location;

(D) withdrawing evaporated refrigerant from said refrigerant evaporatorand condensing withdrawn refrigerant in said refrigerant condenser whilesimultaneously heating said solution thereby; and

(E) passing said subcooled refrigerant from said re- 6 frigerantsubcooler to said refrigerant evaporator for evaporation therein.

10. A method of separating solvent from a solution as defined in claim 9including the step of heating said solution by passing it through thecondensing section of said solution evaporator, and simultaneouslycondensing solvent vapor in said condensing section, by exchanging heatbetween said solution and said solvent vapor.

11. In a method of separating solvent from a solution as defined inclaim 9, said apparatus including a refrigerated flash evaporator havinga condensing section and a solution evaporating section, the additionalsteps including:

(A) flashing additional solvent vapor from said solution by passing itfrom the evaporating section of said solution evaporator to the solutionevaporating section of said refrigerated flash evaporator, therebyproviding solvent vapor in the condensing section thereof; and

(B) evaporating said subcooled refrigerant in the condensing section ofsaid refrigerated flash evaporator, thereby condensing additionalsolvent vapor therein.

12. A method of separating solvent from a solution as defined in claim11, said apparatus including an additional solution evaporator stagehaving .an evaporating section and a condensing section, including thesteps comprising:

(A) flashing additional solvent vapor from said solution by passing itfrom the solution evaporating section of said refrigerated flashevaporator to the evaporating section of said additional flashevaporator to provide solvent vapor in said condensing section thereof;and

(B) passing a cooling medium through the condensing section of saidadditional flash evaporator to condense additional solvent vapor thereinand to heat said solution.

13. A method of separating solvent from a solution as defined in claim 9wherein said apparatus includes a turbine connected for driving acompressor, a power fluid condenser, and a boiler, the additional stepscomprising:

(A) heating and vaporizing power fluid in said boiler;

(B) driving said turbine by supplying power fluid vapor from said boilerto said turbine, thereby operating said compressor to compressrefrigerant vaporized in the refrigerant evaporator;

(C) condensing power fluid vapor discharged from said turbine in saidpower fluid condenser by passing solution from the condensing section ofsaid flash evaporator through said power fluid condenser in heatexchange relation with power fluid vapor therein, thereby heating saidsolution; .and

(D) passing solution heated in said power fluid condenser to theevaporating section of said solution evaporator for evaporation therein.

References Cited UNITED STATES PATENTS 3,234,109 2/1966 Lustenader20322X 3,243,359 3/1966 Schmidt 203-26X 3,021,265 2/1962 Sadtler et al.203-22 X 2,441,361 5/1948 Kirgan 203-473 3,248,305 4/1966 Williamson202-180 3,300,392 1/1967 Ross et al. 203-l1 3,399,118 8/1968 Williamson202173 3,396,086 8/1968 Starmer 202 183 FOREIGN PATENTS 24,930 12/1956Germany.

WILBUR L. BASCOMB, Primary Examiner F. E. DRUMMOND, Assistant ExaminerUS. 01. X.R. 2024173, 235; 203-22, 73, 88,

